watchdog_get_count() never decrements

[aallan]

An issue was raised on pico-feedback around the watchdog_get_count() never decrementing, see raspberrypi/pico-feedback#349. This was an issue with PlatformIO and the @earlephilhower Arduino core.

However, @JamesH65 mentioned that it is also present in the SDK, so raising here.

[kilograham]

This appears to be an RP2040 erratum, which will need to be documented

[kilograham]

we should fix the documentation to say it doesnt work